In a mobile communication system that adopts, for example, the Long Term Evolution (LTE) standard, user data is transmitted via both a handover-source radio base station and a handover-target radio base station at a time of a mobile-station handover between the radio base stations. For example, in downlink communication of transmitting the user data from a core network to a mobile station, the handover-source radio base station transfers the user data transmitted from a host node such as a serving gateway (GW) to the handover-target radio base station. The handover-target radio base station transmits the transferred user data to the mobile station, thereby executing communication of the user data. Note that in uplink communication, the user data is transmitted via a route that is the reverse of that for the downlink communication. By transferring the user data between the radio base stations as stated above, it is possible to realize seamless switching of communication partners in a handover.
Furthermore, in the handover, the user data is protected by encryption using individual encryption keys in individual sections where user data communications are held so as to prevent eavesdropping or tampering of the user data. Specifically, the serving GW encrypts the user data to be transmitted to the mobile station with an encryption key and transmits the encrypted user data to the handover-source radio base station. The handover-source radio base station decrypts the received encrypted data, encrypts the decrypted data with another encryption key, and transfers the encrypted data to the handover-target radio base station. The handover-target radio base station decrypts the received encrypted data, encrypts the decrypted data with still another encryption key, and transmits the encrypted data to the mobile station. The mobile station decrypts the received user data, thereby acquiring the user data from the core network.